Downholes (underground drilling pits or wellbores) are prepared for recovery of hydrocarbon resources including oil and gas (representatively called “oil” sometimes hereafter) from the underground, and tools for the formation or repair of the downholes, such as frac plugs, bridge plugs, cement retainers, perforation guns, ball sealers, sealing plugs, and packers (inclusively referred to as “downhole tools” hereafter), are used and thereafter disintegrated or allowed to fall down as they are in many cases without recovery thereof onto the ground. (Examples of such downhole tools and manners of use thereof are illustrated in, e.g., Patent documents 1-7). Therefore, about the tool of such temporary use, it has been recommended to form the whole or a component thereof constituting a bonding part allowing collapse (i.e. downhole tool member) with a degradable polymer. Examples of such a degradable polymer, may include: polysaccharide, such as starch or dextrin; animal albumin polymers, such as chitin and chitosan; aliphatic polyesters, such as polylactic acid (PLA, typically poly L-lactic acid (PLLA)), polyglycolic acid (PGA), polybutyric acid, and polyvaleric acid; and further, polyamino acids, polyethylene oxide, etc. (Patent documents 1 and 2).
In order to recover hydrocarbon resources (as represented by “oil”) from surrounding formation through the thus-formed downholes, hydraulic fracturing (hydraulic pressure or stream crushing process) is frequently adopted.
Conventional ball sealers as mentioned above as an example of downhole tools used in such a hydraulic fracturing process, have been used in many cases as sealers or fillers for directly filling perforations which were formed in the formation by using perforating gun, etc., for recovery of oils, in order to suppress the entering of excessive amount of work water into the perforations (for example, Patent document 6). Ball sealers used for such purposes generally comprised nondegradable materials, inclusive of nondegradable resins, such as nylon or phenolic resin, or aluminum, if needed, coated with rubbery surface layers for improving the sealing performances, and were of relatively small sizes, e.g., diameters of 16-32 mm (⅝ inch-1.25 inches, as disclosed at col. 2, lines 46-48 of Patent document 6).
In recent years, however, use of ball sealers having larger diameters as a part of members forming frac plugs or frac sleeves (plugs or sleeves for hydraulic fracturing) used in the hydraulic fracturing process is also proposed. More specifically, a high-pressure stream is introduced into an operation region defined by disposing a frac plug formed by a ball sealer together with a ball sealer at a predetermined position along a downhole prepared in advance to urge a water stream in a direction perpendicular to the downhole to fracture the formation, thereby forming perforations for recovering oils (e.g., Patent documents 1 and 2).
Alternatively, there has been also proposed a method of inserting into a downhole a tube (frac sleeve) in which a plurality of ball seats have been disposed at predetermined intervals and successively performing cycles each including supplying and disposition of a ball sealer at a ball seat, and then introducing a high-pressure water stream for fracturing the formation to form a perforation in the frac sleeve (e.g., Patent document 7).
Such ball sealers (which are also called “frac balls”) forming a part of such a frac plug or a frac sleeve, are required to have diameters of, e.g., 25-100 mm (1-4 inches) in many cases, which partly overlap with but are generally larger than the diameters of the above-mentioned ball sealers for directly sealing the perforations. However, if such a large-diameter ball sealer is manufactured by the heat compression molding or injection molding of not a conventional nondegradable material but of a biodegradable resin which is crystalline in general, a sink or a void is developed due to thermal contraction or contraction accompanying crystallization, so that it was difficult to produce such large-diameter balls with good dimensional accuracy required of the frac ball as a member for filling or liquid leakage-preventing member. For this reason, it has been a present state that the manufacture of such frac balls of comparatively large diameters with a biodegradable resin material has been resorted to a sequence of solidification extrusion cutting, which is very complicated and cost-incurring.